1. Field of the Invention
The present invention relates to a display device.
2. Description of Related Art
In general, liquid crystal display devices are structured such that a pair of upper and lower electrode substrates having a transparent electrode formed thereon is bonded with a sealing material applied to a periphery of an image display area on the substrates, and a liquid crystal is filled in the space created by the sealing material and the two substrates. Further, the liquid crystal display devices are classified into an active matrix type and a passive matrix type. The active matrix type liquid crystal display device includes a TFT array substrate having thin film transistors as switching elements arranged in matrix. Then, the TFT array substrate is bonded to the opposing substrate through the sealing material. A liquid crystal is filled in between the TFT array substrate and the opposing substrate.
In a display region of the TFT array substrate, scanning signal lines, display signal lines, and a pixel electrode are formed. The TFT as the switching element is turned ON/OFF in accordance with a scanning signal transmitted on the scanning signal lines. A display signal transmitted on the display signal lines is applied to the pixel electrode through the TFT. Then, if the display signal is applied to the pixel electrode, a display voltage corresponding to the display signal is applied between the opposing electrode and the pixel electrode to drive the liquid crystal.
The scanning signal transmitted on the scanning signal line and the display signal transmitted on the display signal line are supplied from a driving IC. Thus, in a frame region outside the display region, lead-out lines extend from the driving IC to the scanning signal line and to the display signal line. Further, in the frame region, the sealing material and a common line are formed. The common line transmits a common signal for applying a common potential.
Here, if the lead-out line faces opposing electrode across the sealing material or liquid crystal in the frame region, a display signal interferes with a common signal due to a parasitic capacitance thereof. Accordingly, a display quality is deteriorated due to a noise.
To overcome this defect, Japanese Unexamined Patent Application Publication No. 2001-183696 discloses a device where a conductive layer is formed over a lead-out line connected to a display signal line. Then, a ground potential is applied to the conductive layer to shield the display signal line from the opposing electrode to relax interference between signals. Hence, it is possible to prevent the display quality from deteriorating due to the interference between the display signal and a common signal.
Further, in a display device having lead-out lines formed on the right and left sides of the display region, a lead-out line length may locally vary due to a limitation on area of the used driving circuit or frame region. A difference in wiring load (resistance) corresponds to a difference in delay of the scanning signal. Further, a frame region of the display device has crossing points between the lead-out line of the scanning signal line and the lead-out line of the display signal line. The number and area of crossing points may locally vary. These lead to a difference in wiring load of the lead-out line of the scanning signal line, and a delay of the scanning signal varies. Variations of a pixel electrode potential due to the difference in delay look like unevenness of display image to a viewer, and the display quality drops.
Japanese Unexamined Patent Application Publication No. 2005-266394 discloses a countermeasure against the above. According to the technique of this publication, a conductive film is formed to overlap with a portion of the lead-out line of the scanning signal line formed in a frame region. Then, a capacitance is formed between the conductive film and the scanning signal line. Further, as for a lead-out line with a small wiring load outside the conductive film, an overlap between the line and the conductive film is increased. As a result, a wiring load difference between the lead-out lines is reduced to improve the display quality.
There is a space of only about 5 μm between an opposing substrate having an opposing electrode and a TFT array substrate in many cases. Further, an insulating film as a protective film is not generally formed on the opposing electrode. Therefore many corrosion reactions arose in the lead-out lines on the TFT array substrate occur in association with the opposing electrode. Further, the above defects do not occur just after production but mostly occur after a predetermined period. In particular, a display device in a cell phone used outdoors or a display device in an automobile are used under high temperature and high humidity conditions in many cases, so a corrosion reaction resulting from infiltrated water is promoted. Accordingly, in the above display devices, defects easily occur.
Further, the scanning signal line transmits a scanning signal for turning on/off the TFT on the TFT array substrate. Thus, a potential for turning the TFT off is almost always supplied to the scanning signal line. Hence, a potential difference of 10 V or higher occurs between the signal line potential and a common potential applied to the opposing electrode. On the other hand, the display signal line transmits a display signal corresponding to image data. Thus, an average potential of the display signal line is substantially equal to the common potential or different from the common potential by about several V. Accordingly, if the scanning signal line is compared with the display signal line, the scanning signal line has a large potential difference from the opposing electrode. Therefore, defects accompanying the corrosion reaction easily occur in the lead-out line of the scanning signal line. As described above, the display devices of the related art have a problem that a display quality is lowered due to corrosion of the lead-out line.